The present invention generally relates to a developing apparatus and more particularly, to a dry process developing apparatus for use in a copying process such as electrographic copying or the like.
Dry developing processes which have already been put into actual applications at present may be broadly divided into the dual-component or two-component developing process which employs a developing material in a powder form composed of a mixture of toner and magnetic carrier, and the mono-component or one-component developing process which employs a developing material in a powder form composed of only magnetic toner. In the above two-component developing process, although developing and transfer characteristics of the toner itself are superior, there are some problems related to deterioration of carrier and stability of the mixing ratio of the toner to the carrier. Meanwhile, since the one-component developing process is free from any problem due to employment of carrier, it has been a recent tendency for this process to be adopted in many copying apparatuses.
However, with respect to toner, there is a difficult problem in that conditions incompatible with each other, i.e. conditions for electrical conductivity during the development and conditions for electrical insulation during the transfer are required. At the present stage, it is intended to improve the developing characteristics through adoption of various countermeasures by employing electrically insulative magnetic toner having favorable transfer characteristics, without imposing excessive loads on the copying process.
Subsequently, a comparative study will be made between a process in which toner is formed into a thin layer for effecting the developing and another process in which toner is formed into a layer having a thickness to a certain extent through employment of a charging type magnetic toner for the toner.
In the latter thick layer process, if there are no differences between toner particles in the physical properties and surface condition of the toner, the toner can favorably effect triboelectrical charging with respect to a developing sleeve, but part of the toner at the upper part of the layer is hardly charged triboelectrically, thus resulting in a faulty developing. However, if there is a predetermined distribution in the physical properties and/or the surface condition of the toner, the toner at the upper part of the layer can be subjected to the triboelectric charging between the toner particles. In other words, the toner is provided with stable charging characteristics when there is a difference in a triboelectric series, which makes it possible to effect a favorable development. On the other hand, in the former thin layer process, a favorable development may be achieved only if the toner has a difference in the triboelectric series with respect to the developing sleeve, but there is such a disadvantage that the developed images are substantially low in density since the toner layer is thin.
Incidentally, when developing experiments are actually carried out using the thick layer process as described above, it is possible to effect a stable development. The reasons are such that, since the amount of magnetic powder exposed to surfaces of toner particles and amounts of exposed additives such as dye, carbon black, etc. are different according to dispersion, classification, aftertreatment process, etc. during manufacture of toner, there is produced a difference in the triboelectric series between the toner particles, thus resulting in the triboelectric charging of the toner particles to each other. However, when the ratio of inversely charged toner (it is to be noted that originally, individual toner particles are negatively charged by the triboelectric charging thereof with respect to the developing sleeve) in the developing material becomes high, probability of contact between the inversely charged toner and the developing sleeve is increased, with a consequent transfer of the charging polarity to the original negative polarity. In connection with the above, upon separation of only the inversely charged toner, it has been found that the magnetic powder content is high. Since the toner as described above is poor in charge retaining characteristic, and tends to produce a leakage of charge during transportation thereof along a peripheral surface of the developing sleeve, it may be considered that transfer of the charging polarity is easily effected.
Even the thick layer process as described so far has such disadvantages that the developing gap and the magnetic brush bristle height restricting gap employed therein are generally narrow, with a high accuracy being required therefor, while the bristle height restricting gap tends to be clogged by the toner. Moreover, the charging mechanism depends on the state of manufacture of the toner in terms of probability, and it is difficult to reproduce such a state of manufacture of the toner in exactly the same manner. Furthermore, since the toner concentration is at 100%, variation of the toner amount at the developing region is likely to become conspicuous.
In order to overcome the drawbacks as described in the foregoing, it has been considered by the present inventors to mix magnetic carrier into the toner layer on the outer peripheral surface of the developing sleeve.
In FIG. 1, there is shown a basic construction of a conventional developing apparatus employed for the one-component developing process, which generally includes a cylindrical developing sleeve 1 rotatably provided adjacent to a photosensitive surface 5a of a photosensitive or photoreceptor drum 5, for rotation in a direction indicated by an arrow b, a magnet roller 2 sequentially magnetized with S and N poles at its outer peripheral surface and rotatably accommodated in the developing sleeve 1 for rotation in a direction indicated by an arrow a, a bristle height restricting plate 3 fixedly provided above and adjacent to the peripheral surface of the developing sleeve 1, and a toner tank 4 in which toner is contained. By the rotation of the magnet roller 2 in the direction of arrow a and the rotation of the developing sleeve 1 in the direction of arrow b, the toner is transported for circulation along the peripheral surface of the developing sleeve 1 in the direction of arrow b so as to be rubbed against an electrostatic latent image preliminarily formed on the photosensitive surface 5a of the photoreceptor drum 5 driven for rotation in a direction indicated by an arrow c, thereby to develop said electrostatic latent image. In the above case, the amount of toner transported to the developing region A is restricted by the bristle height restricting plate 3.
In the developing apparatus as described above, when the developing is effected by mixing magnetic carrier into the toner layer on the outer peripheral surface of the developing sleeve 1, there was observed by the present inventors a phenomenon that part of the carrier was collected at the upstream side B of the bristle height restricting plate 3 in the direction for transporting the developing material and thereafter, dispersed into the toner contained in the toner tank 4 as shown by an arrow d upon application of the toner transporting force, etc. thereto. In other words, as the developing proceeds, the amount of carrier transported to the developing region A is reduced, with a variation (rising) of toner concentration, or the triboelectric charging between the toner and carrier becomes insufficient, thus making it impossible to obtain developed images of high quality. This also means that the toner concentration can not be set at a predetermined value. Moreover, there is a disadvantage that, if irregularity of the toner concentration takes place in the axial direction of the developing sleeve 1, such an irregular toner concentration is not readily eliminated, since it is difficult to subject the toner and carrier positively to a moving force in the axial direction.